1. Field of the Invention
The present invention relates to a magnetic device used as, e.g., a magnetic field sensing device for sensing a very weak magnetic field in a very small region, and a magnetic sensor using the same.
2. Description of the Related Art
Increases in the density and speed of magnetic recording largely depend upon the improvement of magnetic recording media and the progress of magnetic recording apparatuses, particularly the progress of magnetic heads for performing write and read operations in magnetic recording. For example, with the decreasing sizes and the increasing capacities of the recent magnetic recording media, a relative speed between a magnetic recording medium and a read magnetic head is decreased. Accordingly, as a new type of a read magnetic head capable of extracting a high output even at a low relative speed, a magnetic head called a magnetoresistance effect head (to be referred to as an MR head hereinafter), particularly a giant magnetoresistance effect head (to be referred to as a GMR head hereinafter) has drawn attention. The GMR head uses a large magnetoresistance effect of a multilayered film consisting of a magnetic material and a nonmagnetic material. Various kinds of the GMR heads have been proposed, and a GMR head of type called a spin valve head is considered to be promising among them all.
This spin valve GMR head has a basic structure in which a ferromagnetic layer, a nonmagnetic layer, and a ferromagnetic layer are stacked on an antiferromagnetic layer. The direction of magnetization in the lower ferromagnetic layer is spatially fixed by an exchange interaction acting between this ferromagnetic layer and the antiferromagnetic layer. The magnetization in the upper ferromagnetic layer weakly interacts with the magnetization in the lower ferromagnetic layer through the nonmagnetic layer. However, the upper ferromagnetic layer can be antiferromagnetically coupled with the lower ferromagnetic layer by properly selecting the thickness of the nonmagnetic layer. That is, when an external magnetic field is zero, the magnetization in the upper ferromagnetic layer couples with the magnetization in the lower ferromagnetic layer in opposite directions. Since this antiferromagnetic magnetic coupling is weak, the direction of the magnetization in the upper ferromagnetic layer is readily reversed if an external magnetic field is applied to the direction of the magnetization in the lower ferromagnetic layer. That is, under an external magnetic field, the directions of magnetization in the upper and the lower ferromagnetic layers are the same.
It is known that the electric resistance of a multilayered film consisting of a ferromagnetic layer/nonmagnetic layer/ferromagnetic layer structure depends upon the relative magnetization direction in the upper and the lower ferromagnetic layers. This is so because in a magnetic material the scattering of conduction electrons depends upon the spin magnetic moments of the electrons. Since the relative magnetization direction in the multilayered film described above depends upon an external magnetic field, the electric resistance of the film strongly depends upon the external magnetic field. This phenomenon is called a giant magnetoresistance effect. The spin valve GMR head uses this giant magnetoresistance effect and reads out magnetically recorded data.
Although the magnetic head using the giant magnetoresistance effect has excellent characteristics, it also has several drawbacks. The main cause of these drawbacks is that the multilayered film having the giant magnetoresistance effect is a metal multilayered film with a low electric resistance, and so it is necessary to increase the density of a current made to flow through the multilayered film in order to obtain a sufficient output voltage. When the density of a current made to flow through the device is thus increased, the device generates heat, electromigration occurs, or a magnetic field is generated by the current, thereby making the device operation unstable.
A larger magnetoresistance effect is expected when a current is made to flow vertically in the multilayered film. However, since the absolute value of the resistance is very small in this direction, no practical device can be obtained from the present GMR head structure.
As described above, in the conventional magnetic heads using the magnetoresistance effect, the density of a current made to flow through the multilayered film must be increased to obtain a sufficient output voltage. When the current density is thus increased, the device generates heat, electromigration occurs, or a magnetic field is generated. It is therefore necessary to eliminate these problems. Also, a magnetic head structure is demanded in which a sufficient output voltage can be obtained with a low current density even when a current is made to flow vertically in the multilayered film, in which case a larger magnetoresistance effect can be expected.